[Bug ada/95959] New: Error in conversion from UTF16 to UTF8

["simon at pushface dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95959

            Bug ID: 95959
           Summary: Error in conversion from UTF16 to UTF8
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ada
          Assignee: unassigned at gcc dot gnu.org
          Reporter: simon at pushface dot org
  Target Milestone: ---

Created attachment 48799
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48799&action=edit
Demonstration

There's an error in converting from UTF16 to UTF8 for code points in
U+10000 to u+10FFFF (which require 4 UTF8 bytes).

The attached demonstration shows this by taking a UTF8 character
(Clef, U+1D11E), converting to UTF16, and converting back to UTF8,
which should round-trip back to the same character, but doesn't. The
third byte of the final UTF8 is wrong

$ ./utftest 
Codepoint: 16#1D11E#
 UTF-8: 4: 2#11110000# 2#10011101# 2#10000100# 2#10011110#
UTF-16: 2: 2#1101100000110100# 2#1101110100011110#
 UTF-8: 4: 2#11110000# 2#10011101# 2#10010000# 2#10011110#
Bug

The attached patch corrects the problem.